Water soluble polymers formed from monoethylenically unsaturated dicarboxylic acid monomers, herein called "dicarboxylic acid monomers" and optionally other water soluble monoethylenically unsaturated monomers, herein called "optional other water soluble monomers" are useful as scale inhibitors, deflocculants, dispersants, in water circulating systems and as incrustation inhibitors, builders, anti-filming agents, sequestering agents, and dispersants in detergents and cleaning formulations.
Several techniques for preparing these water soluble polymers by free radical addition polymerization in aqueous systems are known. However, a common problem in forming these polymers is that the dicarboxylic acid monomer, such as maleic acid, is slow to react in comparison to the other monomers, such as acrylic acid, in free radical addition polymerizations. To compensate for this slow reactivity of the dicarboxylic acid monomer, one process which has been developed for producing these polymers is described in U.S. Pat. No. 4,659,793 to Yang, herein called "Yang". Yang discloses a process where a "heel" or solution of partially neutralized dicarboxylic acid monomer and water is first formed. The polymerization is then conducted by slowly feeding the non-dicarboxylic acid monomer and initiator into the heel, and is carried out in the presence of at least one metal ion. However, in Yang, the preferred feed time of the non-dicarboxylic acid monomer and initiator is 5 to 7 hours, and the process is limited to producing polymers containing less than 55 weight percent dicarboxylic acid monomers.
German Patent Application Number DE 40 08 696 A1 to Denzinger et al., herein called "Denzinger" discloses another "heel" process for making polymers that contain greater than 65 weight percent dicarboxylic acid monomers. In Denzinger, water soluble ethylenically unsaturated monomer and initiator are added to a heel of partially neutralized dicarboxylic acid monomer. Denzinger requires the final overall degree of neutralization of acid groups on all monomers be from 52 to 70 percent and preferably 55 to 65 percent to obtain less than 1.5 weight percent residual dicarboxylic acid monomer based on the total weight of polymer product. However, a major disadvantage to Denzinger is that the time for feeding the initiator and ethylenically unsaturated monomer in the examples is about six hours and the hold time after the completion of the feeds is about one to two hours for a total polymerization reaction time of 7 to 8 hours.
Consequently, it is an aim of the present invention to produce water soluble polymers containing about 50 weight percent to 100 weight percent dicarboxylic acid monomers where the amount of unreacted dicarboxylic acid monomer in the polymer product is less than about 1.5 weight percent based on the total weight of polymer product.
It is another aim of this invention to shorten the polymerization reaction time of the process to less than about 6 hours and still obtain a polymer product with less than about 1.5 weight percent unreacted dicarboxylic acid monomer.